1. Field of the Invention
The present invention relates to a film feeding mechanism, for a camera, an image reader or the like, which uses a film having one perforation formed at a specified location relative to each frame of the film, or uses a film having a plurality of perforations formed at specified locations relative to each frame thereof with unequal intervals therebetween as a whole.
2. Description of the Related Arts
In a conventional 35 mm film, eight perforations are formed for every frame along a longer side edge of the film. In cameras using this 35 mm film, generally, a sprocket having eight teeth to engage with the perforations of the film is employed, and the frame advance of the film is controlled by detecting one rotation of the sprocket during the film transport.
Also, there have been provided a camera in which the frame advance of the film is controlled by counting the eight perforations with a photo-interrupter.
Meanwhile, a film of a new standard in which one perforation is formed at a front end and a rear end respectively with respect to each frame of the film so that the perforations are arranged relative to each frame thereof with unequal intervals therebetween as a whole has been proposed. In case that such a film is used, an attempt to control the frame advance of the film by employing such a common sprocket as mentioned above could result in too complex a structure to be practical, even if possible. Thus, for cameras using this type of film, it is common practice to employ a photo-interrupter or other optical detector means to control the frame advance of the film.
The photo-interrupter system has a problem that not only the photo-interrupter itself is high-priced, but also accessory components such as a control circuit thereof are also costly, as compared with the sprocket system. To deal with this problem, Japanese Laid-Open Patent Publication No. 7-319017 has proposed a film feeding mechanism in which the film transport is controlled without employing a photo-interrupter but with a rotatable sprocket the rotation of which follows the film advance, as in the case where a 35 mm film is used. In the film transport system as disclosed in this publication, the sprocket has two teeth in correspondence to two perforations which are formed per frame of the film, and the sprocket is so arranged as to make one rotation while the film advances one frame, in which arrangement the frame advance of the film is controlled by detecting one rotation of the sprocket.
However, in this film feeding mechanism, because the sprocket is rotatable, there is a possibility that the positions of the teeth of the sprocket do not coincide with the positions of the perforations of the film during the film transport, especially during an initial stage at which the film is forwarded, resulting in an incorrect frame advance of the film.
Such a problem not only occurs to the aforementioned new standard film, but also may occur to those films in which perforations are not arranged uniformly with a short pitch therebetween, unlike 35 mm films, i.e., to other new standard films possibly proposed in future in which one perforation, or three or more perforations are formed per frame of the film.